Stage 1 · Lesson 4 of 10
Collections, generics and LINQ
List<T>, Dictionary<TKey, TValue> and HashSet<T> hold most of your data. (the <T> part) let the compiler check what goes into them. , short for Language Integrated Query, is how C# code filters, maps, groups and sorts data.
For example, orders.Where(o => o.Paid).GroupBy(o => o.CustomerId) reads like the question you are asking. The key idea is deferred execution: a LINQ query does nothing until you actually loop over it with foreach, ToList or Count. That matters later, because Entity Framework Core turns the same LINQ into .
Common traplooping over one query several times, which repeats the work each time, or calling ToList too early on a database query and filtering millions of rows in memory.
A question to check yourself, an AI practice session and 1 cheat sheet entry come with the full roadmap. Unlock
Resources for this stage
Start with
- GuideFreeLearn C#
Microsoft · Beginner
Official C# tutorials from the language basics through classes, LINQ and async
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